Batteries used in EV (Electric Vehicle), HEV (Hybrid Electric Vehicle) and so forth are made for high voltage. In a power supply system having this type of battery, semiconductor integrated circuit devices such as a battery monitor AFE (Analog Front End) for controlling charging/discharging and monitoring of the battery are provided.
This type of semiconductor integrated circuit device generally has a so-called conditioning circuit having a selective charging/discharging function of a battery cell, the conditioning circuit preventing, by uniforming voltages of the battery cells, significant degradation of available electric energy as considered as a battery (as a whole) as compared with a total of discharge capacities which the battery cells have.
As cell balancing technology (uniforming each battery cell voltage) for this type of battery, there are technology of uniforming charges of cells next to each other by a back boost converter etc. (e.g., see Patent Literature 1), technology of injecting charges to a specific battery cell from the whole battery using a converter (e.g., see Non-Patent Literature 1), technology of using a switching element and a capacitor to which technology of charge pump is applied (e.g., see Patent Literature 2), and so forth.
Patent Literature 1 is about a method of uniforming charges between two cells next to each other by voltages of both of the cells. According to this switching operation for alternately opening and closing a switch connected to upper and lower cells with the same pulse width interposing a restricted band, energy of one of the cells is stored in an inductor connected at a middle point of the two cells and then the energy is transferred to the other cell.
When the voltages of the two cells are different by a certain amount that is determined by a circuit constant and a pulse width or more, the energy flows from a cell at a higher voltage to a cell at a lower voltage; when the difference is the certain amount or less, flows of the energy from a cell at a higher voltage to a cell at a lower voltage and from a voltage at a lower cell to a cell at a higher cell are generated at the same time and a rate of the flows depends on the voltages of the cells.
By virtue of this phenomenon, as the voltages of the two cells gradually come close and then the voltage of each cell becomes completely the same. In this case, the two flows of energy coincide. When the number of the cells is three or more, two switches are needed for one inductor with respect to neighboring two cells. When a battery is formed of N cells, N−1 inductors and 2N−2 switches are needed.
In addition, the active cell balance system using a converter described in Non-Patent Literature 1 is a system for extracting energy from between an electrode on the higher-potential side, that is, a positive electrode of the battery cells serially connected and an electrode on the lower-potential side. It is a system for charging energy extracted from the whole battery to battery cells.
Moreover, the technology described in Patent Literature 2 composes a main part by a switch and a capacitor in the same manner as the charge pump. Charges once stored in the capacitor are transferred to another cell etc. in the method.